easy to avoid SHO ( sub har osc ) if you run ave current mode loop - or simple Vout control loop

 

Sorry, I don't know how to enable email notifications and I lost these last messages.

As usual I don't understand what does Papabravo tries to explain. The input is a constant voltage at about 320V. The output must be regulated between about 100V and 300V. Therefore a step down converter is needed, and it can operate with a duty cycle in the range 30% to 100%. I think it's easy to understand...
Then: the converter operates in CCM (L = 400uH; f = 100kHz) and at 99% duty cycle the inductor's current shape is almost flat. Why are you talking about "inductor's time-to-discharge"?

As already mentioned the power stage has already been extensively tested in open loop, it works perfectly, also because it is trivial (and those who question this thing are likely to have some confusion about how a step down topology works).

Let's go ahead:
output voltage and inductor current are sent to a microprocessor that calculates the power: at the beginning I thought to close the loop directly with the onboard microprocessor using its output to generate the PWM for driving the mosfet. But, because the microprocessor is already engaged in other functions, I have experienced some problems regarding critical tasks like pulse-by-pulse current limitation. Therefore I have decided to use it just to give the analog reference to an i.c. controller, and leave to the latter the task of generating the PWM and implementing the pulse-by-pulse protection.

Then the power loop is closed through the microprocessor AND the controller, but that makes things complicated for me:

Should the controller operate in open loop as a simple block of the power loop? (a trivial PWM generator with current-limit features)?
Or should it work with its own voltage loop closed (voltage mode)?
Or maybe with both voltage and current loop closed (current mode)? (and how to deal with slope compensation?)

In any case what controller should I better choose? What about UC35705 for voltage mode (with current limit features), and UC3842 for current mode?